Material handling implements such as tractors, bulldozers, and the like, frequently employ multi-speed powershift transmissions which are shiftable through a plurality of sequenced gear ratios. Gear ratio selection is effected by selective fluid pressurization of hydraulic clutches. In such a transmission, a plurality of powershift hydraulic clutches are typically provided where an engagement of each clutch by fluid pressurization provides a respective one of the transmission's multi-speeds. Electrically or manually controlled hydraulic valves are typically employed for supplying pressurized fluid to the hydraulic clutches.
To reduce operator fatigue, powershift transmissions allow the operator to readily effect a change in gear ratios through manual manipulation of a gear shift lever without disconnecting the power source from the transmission attendant to each change or shift in gear ratio. Instead, powershift transmissions typically include a control system having suitable controls for operating the hydraulic valves to effect shifting of the transmission by sequentially pressurizing one of the transmission's hydraulic clutches (for engagement) while relieving fluid pressure in a previously engaged hydraulic clutch. Naturally, efficient operation of the tractor or other implement is promoted by convenient transmission gear ratio selection.
As will be appreciated by those familiar with the art, it is desirable to provide a transmission which can be operated such that powershifting between gear ratios is effected as smoothly as possible. Such ends may be effected through relatively precise sequential operation of the transmission's hydraulic clutches. Although fluid pressurization and depressurization of the clutches involved in a shift is effected quite quickly, operation of the associated hydraulic valves and full clutch engagement or disengagement does change depending on several variable factors.
Because fluid pressurization of an on-coming hydraulic clutch is not instantaneously effected, control arrangements for such transmissions typically function such that initial fluid pressurization of the on-coming hydraulic clutch commences prior to relief of fluid pressure on the off-going hydraulic clutch. Prior commencement of fluid pressurization for the on-coming clutch is required because of inherent time delays involved in actuating the on-coming clutch. To further complicate the problem, these inherent time delays vary depending upon such factors as: response time of the particular electric valve controlling fluid flow to the on-coming clutch; clutch fill period; fluid pressure fluctuations involved in actuation of the clutch; linear movement of a clutch piston or the like against a resilient force; clutch wear; and, compression of a series of interleaved plates comprising the on-coming clutch.
The time period between initial full engagement of the on-coming clutch and initial disengagement of the off-going clutch is sometimes referred to as a "critical time" period. Preferably, it is during this critical time period that pressure levels of the on-coming and off-going clutches are concurrently modulated such that as the on-coming clutch is going pressurized for full engagement the off-going clutch is being depressurized toward full disengagement.
Minimization of "shift shock" or "jerk" between gear ratios is the desired goal in configuring transmission control systems. Shift shock is a function of the elapsed time between disengagement of the off-going clutch and initial full engagement of the on-coming clutch. As will be appreciated, if the off-going clutch is disengaged too quickly following initial pressurization of the on-coming clutch, both clutches can be momentarily disengaged, When operated under load, the implement will quickly decelerate between gear ratio shifts or changes. As such, when the on-coming clutch, used to effect the desired gear ratio change, is sufficiently pressurized to transfer torque, the implement is accelerated with a significant shift shock if the critical time between clutch operations is too long. On the other hand, if the off-going clutch is disengaged after the on-coming is fully engaged such that the critical time period is too short, both clutches may be momentarily engaged resulting in objectionable noise generation and possible clutch and transmission damage.
In the past, control systems for powershift transmissions engage and disengage the hydraulic clutches at preset timed intervals regardless of shift quality or performance. While such arrangements can perform satisfactorily, these systems fail to quantify shift performance, do not compensate for clutch wear, or hydraulic fluid temperatures, or a myriad of other variables which effect shift quality and performance.
It is, therefore, desirable to provide a control system for a transmission having a plurality of selectively operable hydraulic clutches wherein the control system monitors transmission shift performance for each gear ratio change or transmission shift and thereafter automatically self adjusts the shifting sequence to optimize for smooth and shock free shifting the next time that same shift is effected.